1. Field of the Invention
The present invention relates to heat exchangers which, through convection or radiation, are designed to absorb varying amounts of energy at their heat transmission walls.
Heat exchangers of this type are presently employed, for instance, in radiation space heaters and with collectors for thermal solar energy use having line or spot focusing systems. Herein, under heat exchangers is to be understood, are hollow members of any desirable cross-sectional configuration which accommodate the flow of fluids, in which the cross-section can vary over the length of the heat exchanger.
The load capacity of heat exchangers, radiation absorbers, or other components which are exposed to high temperatures, is normally extensively curtailed when a non-uniform heat load is present, which leads to varying temperatures along the walls of the heat exchanger.
The permissible load on such components is then no longer primarily limited by the maximum temperature which can be taken by the materials over the coarse of time, but by the stresses which are generated by the temperature differences. In contrast with stresses which normally result from the actual operating load and which, as a rule, are invariable, the stresses induced by a non-uniform heat input can be reduced in many instances.
The heat exchanger surfaces facing towards the direct radiation are subjected to a clearly higher thermal load than is the rear wall of the heat exchanger. However, the actual load on the tube is thereby not to be viewed alone in the high wall temperatures of the tube side which is under direct radiation. They are, above all, the two minima between the sector under direction radiation and the sector under radiation from the reflection wall, and the resultant extensively varying temperature gradients over the circumference of the tube, which occasion the critical stresses.
2. Discussion of the Prior Art
When the temperatures along the circumference of a heat exchange tube rise and fall linearly between a minimum value and a maximum, then the thermally induced stresses on the tube can be largely eliminated through a suspension arrangement which is free from forces and moments, although this will not adequately solve the problem, especially in the area of minimum temperatures.
There are areas of applications such as solar tower power plants, in which the heat exchanger is exposed to a large number of thermal load changes, such as are encountered over the day-and-night cycle and from irregular clouding over of the sun. Obviously, these rising and falling cycles will develop stresses on the heat exchanger and restrict the performance of the materials thereof.
Besides the above-mentioned suspension of the heat exchanger, precurved constructions to reduce thermally induced stresses, and which can more effectively control the distortions.
As a rule, it is attempted to reduce thermal stresses resulting from thermal cycling by timed stretching of the heating and cooling phases, which often requires complex constructional measures for the holding of the temperatures.
A very effective measure for reducing the temperature differences between the radiation side and the rear side is the use of ceramic shields which, when arranged in front of the heat exchangers, will restrain a part of the direct radiation. This protective shield, however, is accompanied by relatively high temperatures of the shield, and with these high temperatures also radiates a correspondingly increased heat quantity from the absorber.